1. Field of the Invention
The present invention relates to a tubular radiation absorbing device for solar thermal applications, especially for a parabolic trough collector in a solar power plant, which comprises a central tube and a glass tubular jacket surrounding the central tube so as to form a ring-shaped space between the tubular jacket and the central tube.
2. Related Art
Tubular radiation absorbing devices or absorber pipes are used in parabolic trough collectors to utilize solar radiation. The solar radiation is concentrated by a tracking mirror on a tubular radiation absorbing device and converted into heat. The heat is conducted away by a heat-carrying medium passing through the tubular radiation absorbing device and is used directly as process heat or converted into electrical energy.
This sort of tubular radiation absorbing device typically comprises a coated central tube and a glass tubular jacket around it. The ring-shaped space between the tubes is evacuated. In operation a heat carrier fluid, especially an oil, is pumped through the central tube.
This sort of absorber tube is described, e.g., in DE 102 31 467 B4. A glass-metal transitional element is arranged at the free end of a glass tubular jacket. The central tube and the glass-metal transitional element are connected with each other so that they are movable longitudinally relative to each other by means of at least one expansion compensating device.
The operating temperature range of the tubular radiation absorbing device is between 300° C. and 400° C. for solar power generation.
Free hydrogen, which is dissolved in the heat carrier medium, is generated by aging of the heat carrier fluid. This hydrogen arrives in the evacuated ring-shaped space between the central tube and the glass tubular jacket by permeation through the central tube. The permeation rate increases with increasing operating temperature, which is between 300° C. and 400° C., so that the pressure in the ring-shaped space rises. This pressure increase leads to increased heat losses and to a reduced efficiency of the tubular radiation absorbing device.
Suitable measures must then be taken to maintain a vacuum in the ring-shaped space. One measure that is taken to remove hydrogen is to combine it with a suitable material.
Getter material, which combines or reacts with the hydrogen gas that penetrates through the central tube into the ring-shaped space, is arranged in the ring-shaped space to maintain the vacuum. When the capacity of the getter material is exhausted, the pressure rises in the ring-shaped space until the partial pressure of the free hydrogen in the ring-shaped space reaches equilibrium with the hydrogen dissolved in the heat carrier medium. The equilibration pressure of the hydrogen in the ring-shaped space amounts to between 0.3 mbar and 3 mbar in the known absorber tubes. There is an increase in heat conduction in the ring-shaped space because of the presence of hydrogen in it. The heat losses due to heat conduction are about five times higher compared to air, i.e. clearly higher than with an absorber tube that has not been evacuated.
A getter arrangement is described in WO 2004/063640 A1, in which a getter strip is arranged between the central tube and the tubular jacket in the ring-shaped space. This arrangement has the disadvantage that the strip is in a region, which can be exposed to direct radiation. The getter strip can be heated especially by radiation coming from the mirror that misses the central tube or strikes it but is largely reflected from it. Since the getter strip is nearly thermally isolated from the central tube and the tubular jacket in a vacuum, the temperature of the getter strip can vary greatly with varying irradiation conditions. Because the getter material with a predetermined loading degree has a temperature-dependent equilibrium pressure (equilibrium between gas desorption and adsorption), temperature fluctuations of the getter material lead to undesirable pressure fluctuations. The temperature of the tubular jacket greatly increases after consumption of the getter material and the absorber tube becomes unusable.